Particle detection apparatus

ABSTRACT

There is provided a particle detection apparatus ( 30 ) comprising:
         a channel ( 32 ) including an inlet and at least one channel wall, the inlet permitting light to be introduced into the channel ( 32 ), the or each channel wall being arranged to define a channel path through which light may propagate;   a light source ( 34 ) configured to introduce light into the channel ( 32 ) via the inlet, the channel ( 32 ) being shaped to guide the light to propagate along the channel path for illuminating a particle or a plurality of particles located in the channel path; and   a monitoring device ( 36 ) configured to detect scattered light that is created by the illumination of the or each particle by the guided light and that leaves the channel ( 32 ) by passing through the or each channel wall.

This application is a national phase of International Application No.PCT/EP2015/070647 filed Sep. 9, 2015 and published in the Englishlanguage.

This invention relates to a particle detection apparatus.

It is known to use various microscopy techniques to detect and study thebehaviour of nanoparticles at the nanoscale. Examples of such microscopytechniques include electron microscopy and fluorescent microscopy.

According to a first aspect of the invention, there is provided aparticle detection apparatus comprising:

-   -   a channel including an inlet and at least one channel wall, the        inlet permitting light to be introduced into the channel, the or        each channel wall being arranged to define a channel path        through which light may propagate;    -   a light source configured to introduce light into the channel        via the inlet, the channel being shaped to guide the light to        propagate along the channel path for illuminating a particle or        a plurality of particles located in the channel path; and    -   a monitoring device configured to detect scattered light that is        created by the illumination of the or each particle by the        guided light and that leaves the channel by passing through the        or each channel wall.

In a preferred embodiment of the invention the inlet may be configuredas an open inlet.

The inlet being an open inlet allows the light source to directlyintroduce light into the channel via the open inlet, i.e. the lightenters the channel without passing through an intermediate object suchas a channel wall.

Such an open inlet may be configured to permit the or each particle tobe introduced into the channel.

In further embodiments of the invention the inlet may be configured as alight incoupling inlet.

The particle detection apparatus of the invention permits opticaldetection of particles that are freely diffusing in a fluid (e.g. liquidor gas) present in the channel path. In particular, the configuration ofthe particle detection apparatus of the invention permits use ofcoherent and/or incoherent light scattering to detect very smallparticles, especially those in the sub-100 nm range.

By configuring the channel and monitoring device in the manner set outabove, any light that is not scattered by the or each particle staysguided along the channel path so that only the scattered light isdetected by the monitoring device. This may be achieved by, for example,the scattered light leaving the channel through the or each channel wallat a non-zero angle to a guided direction of the guided light. This inturn provides excellent signal to background and signal to noise ratiosand thereby enhances the detection of the or each particle, thuspreventing the detected scattered light from being overwhelmed by directdetection of the residual scattering of the illuminating guided light.

In addition the configuration of the particle detection apparatus of theinvention allows the or each particle in the channel path to stayilluminated by the guided light and thereby remain in the imaging planeand not diffuse out of focus. Keeping the or each particle in theillumination plane of the guide light not only obviates the need forimmobilisation of particles in a restricted volume, such as thatperformed in cryogenic electron microscopy, and thereby results in aless complex and cheaper particle detection apparatus, but also providesa prolonged detection period that permits enhanced real-time tracking ofthe or each particle and increases the obtainable amount of informationabout the or each particle.

Furthermore the configuration of the particle detection apparatus of theinvention enables coherent illumination of a plurality of particles sothat, when the plurality of particles approach each other, any resultantnear-field interference effect results in enhancement of the detectionsensitivity of the particle detection apparatus of the invention.

The improved detection capabilities of the particle detection apparatusof the invention as set out above not only obviates the need for aspecialised monitoring device to detect the or each particle and therebypermits use of simpler and cheaper monitoring devices, such as anoptical microscope, a smartphone camera or simpler photo-detectionelectronics, but also permits detection of the or each particle underambient conditions, instead of specific conditions as required bycryogenic electron microscopy.

The monitoring device may be configured to detect the scattered light tostudy the or each particle in different ways, examples of which are asfollows.

The monitoring device may be configured to measure the coherentscattering intensity of the scattered light, the incoherent scatteringintensity of the scattered light, the spectrum of the scattered light,the distribution of the scattered light over a plurality of directionsand/or the dynamic motion of one or more particles, preferably theEinstein-Stokes diffusion constant of the or each particle.

Measurement of the scattering intensity of the scattered light permitsthe study of particle interaction. For example, measurement of thescattering intensity of the scattered light permits the study ofparticle binding and unbinding events through detection of quadraticchanges in the scattering intensity, the spectral response or thediffusion constant of the or each particle.

In addition the simultaneous measurement of the scattering intensity andthe Einstein-Stokes diffusion constant of the or each particle allows anaggregate of particles to be distinguished from a single larger particleeven if they both exhibit similar scattering intensities.

The monitoring device may be configured to track the or each particle'smotion through detection of the scattered light. Such tracking of the oreach particle's motion permits the study of the hydrodynamic behaviourof the or each particle.

The monitoring device may be configured to measure an emission spectrumof the or each particle. This permits identification of the or eachparticle on the basis of its spectral features.

The monitoring device may be configured to detect fluorescent light thatis created by the illumination of the or each particle by the guidedlight and that leaves the channel by passing through the or each channelwall.

The monitoring device may be configured to detect coherently scatteredlight and/or incoherently scattered light, and optionally detect thespectrum of the coherently scattered light and/or incoherently scatteredlight, that is created by the illumination of the or each particle bythe guided light and that leaves the channel by passing through the oreach channel wall.

Optionally the monitoring device may be configured to track the or eachparticle's motion through detection of the coherently scattered lightand/or incoherently scattered light.

The configuration of the monitoring device to detect scattered lightpermits the use of metallic, semiconductor or organic contrast agents toenhance the polarizability of the or each particle and thereby enhancethe detection sensitivity of the particle detection apparatus.

Moreover the configuration of the monitoring device to detect bothscattered and fluorescent light permits simultaneous measurement of thescattering and fluorescence in order to, for example, count the numberof fluorescent particles via stepwise bleaching, or to measure anemission spectrum of the or each particle to identify the or eachparticle on the basis of its spectral features.

The choice of channel used in the particle detection apparatus of theinvention may vary depending on a range of factors, such as particlesize, chemical composition, equipment availability and so on.

The size of the channel may vary depending on the size of the or eachparticle to be detected. For example, the or each channel wall mayenclose a sub-wavelength bore that defines the channel through whichlight may propagate, and/or the channel may be arranged to convey atleast one particle that is smaller than the wavelength of the guidedlight.

The channel may be formed in different ways to enable detection of thesize of the or each particle to be detected. The channel may be formedin or as

-   -   a waveguide;    -   a chip-based platform, optionally a lithographically formed        chip-based platform; or    -   a capillary.

The channel may be formed in or as an optical fibre. The optical fibremay be a single-mode optical fibre. The use of such an optical fibreimproves the manner in which the light is guided along the channel path,and thereby improves the resultant illumination of the or each particlelocated in the channel path and the subsequent scattering of light.

The material of the optical fibre may vary. For example, the opticalfibre may be made from silica or polymer.

The particle detection apparatus may include a driving mechanism fordriving the or each particle to flow along the channel path. In oneexample of such a driving mechanism, electrodes may be incorporated intothe channel to permit use of an electrophoretic force to steer the oreach particle along the channel path. In another example of such adriving mechanism, the driving mechanism may be external to the channeland may be configured to generate an electric field so as to provide anelectrophoretic force to steer the or each particle along the channelpath.

Optionally the driving mechanism may be configured to drive the or eachparticle to overcome its or their thermal Brownian motion when drivingthe or each particle to flow along the channel path. The remotemanipulation of the or each particle in a manner that is strong and fastenough to overcome its or their thermal Brownian motion is made possibleby the availability of active feedback provided through the use of theparticle detection apparatus to track the motion of the or eachparticle.

The monitoring device may include at least one light detector and/or animaging device configured to process the detected light so as to createan output signal corresponding to the or each particle. The outputsignal may be static or dynamic. The creation of the output signalpermits visualisation of the or each particle.

The particle detection apparatus may include a plurality of channels,each channel including an inlet and at least one channel wall, eachinlet permitting light to be introduced into the respective channel, theor each channel wall being arranged to define a respective channel paththrough which light may propagate. Optionally the monitoring device maybe configured to simultaneously detect scattered light that is createdby the illumination of the or each particle by the guided light and thatleaves each channel by passing through the or each corresponding channelwall.

The particle detection apparatus may include a plurality of lightsources, the plurality of light sources being configured to introducelight into the channel via the inlet. Optionally the plurality of lightsources may be configured to simultaneously introduce light into thechannel via the inlet.

The particle detection apparatus may include a plurality of monitoringdevices, the plurality of monitoring devices being configured to detectscattered light that is created by the illumination of the or eachparticle by the guided light and that leaves the channel by passingthrough the or each channel wall. Optionally the plurality of monitoringdevices may be configured to simultaneously detect scattered light thatis created by the illumination of the or each particle by the guidedlight and that leaves the channel by passing through the or each channelwall.

According to a second aspect of the invention, there is provided amethod of detecting at least one particle, the method comprising thesteps of:

-   -   providing a channel including an inlet and at least one channel        wall, the inlet permitting light to be introduced into the        channel, the or each channel wall being arranged to define a        channel path through which light may propagate;    -   providing a particle or a plurality of particles in the channel        path;    -   introducing light into the channel via the inlet, the channel        being shaped to guide the light along the channel path and        thereby illuminate the particle or the plurality of particles        located in the channel path; and    -   detecting scattered light that is created by the illumination of        the or each particle by the guided light and that leaves the        channel by passing through the or each channel wall.

It will be understood that the features of the particle detectionapparatus according to the first aspect of the invention applies mutatismutandis to the method of detecting at least one particle according tothe second aspect of the invention. It follows that the correspondingadvantages described above with reference to the first aspect of theinvention applies mutatis mutandis to the second aspect of theinvention.

A preferred embodiment of the invention will now be described, by way ofa non-limiting example, with reference to the accompanying drawings inwhich:

FIG. 1 shows schematically a particle detection apparatus according toan embodiment of the invention;

FIG. 2 shows the light intensity profile in an optical fibre that formspart of the particle detection apparatus of FIG. 1;

FIGS. 3 to 5 illustrate the scattering intensity as a function ofposition when an aqueous suspension of dielectric latex nanoparticlesare conveyed along a channel path of the particle detection apparatus ofFIG. 1;

FIG. 6 illustrates the tracking of the positions of single cowpeachlorotic mottle viruses with time using the particle detectionapparatus of FIG. 1;

FIG. 7 illustrates the tracking of the positions of dielectric latexnanoparticles and single cowpea chlorotic mottle viruses with time usingthe particle detection apparatus of FIG. 1;

FIG. 8 illustrates a theoretical comparison of scattering cross-sectionversus diffusion constant for spherical particles of refractive indicesand for aggregates of spherical particles;

FIGS. 9 to 11 illustrate the simultaneous tracking of fluorescence andscattered light from dielectric latex nanoparticles using the particledetection apparatus of FIG. 1;

FIG. 12 shows a measuring cell in the form of an optical fibre with ahollow channel in a top view on a front side of the fibre;

FIG. 13 is a micrograph showing a fracture surface of a thin tube fromwhich the measuring cell is produced (FIG. 6; reference numeral 89);

FIG. 14 shows a diagram with radial radiation intensity curves of theguided light in two different examples of the optical fibre and fordifferent wavelengths of the guided light;

FIG. 15 shows a simulation of the maximum intensity measured in thelight-guiding hollow channel in dependence upon the diameter of thehollow channel (Poynting vector);

FIG. 16 shows method steps for producing a measuring cell; and

FIG. 17 illustrates the use of the particle detection apparatus of FIG.1 to track gold nanoparticles that are electrophoretically actuatedinside the optical fibre of FIG. 2.

A particle detection apparatus according to an embodiment of theinvention is shown in FIG. 1 and is designated generally by thereference numeral 30.

The particle detection apparatus 30 comprises a channel 32, a lightsource 34 and a monitoring device 36.

The channel 32 is formed in a single-mode, optical fibre 38 that is madefrom silica and includes a sub-wavelength bore. The channel 32 includesan inlet and a channel wall. The inlet permits light to be introducedinto the channel 32. The channel wall encloses a 200 nm tubular borethat defines a channel path through which light may propagate. FIG. 2shows a mode profile of the optical fibre 38 at 670 nm.

The optical fibre 38 may instead be made from polymer. In otherembodiments of the invention, the channel 32 is formed as a waveguide 40fabricated on a chip-based platform by way of lithography, as shown inFIG. 1, or as a capillary.

In use, a plurality of particles is conveyed along the channel path byway of a capillary force or by application of an external pressure.

The light source 34 is configured to introduce light into the channel 32via the inlet. In the embodiment shown, the light source 34 is a laser.In use, the tubular shape of the channel 32 guides the light topropagate in a single mode along the channel path and thereby illuminateeach particle located in the channel path.

The monitoring device 36 includes an objective OBJ, which is configuredto result in an overall magnification of 400× and an effective field ofview of more than 200 μm, a dichromatic beam splitter DBS, a knife-edgemirror KEM, and a scientific complementary metal-oxide-semiconductor(sCMOS) camera with a maximum frame rate of 3.5 kHz for a 6 pixels by1024 pixels area. The objective OBJ is positioned outside the channelwall to collect light that leaves the channel 32 by passing through thechannel wall. The dichromatic beam splitter DBS and knife-edge mirrorKEM are positioned between the objective OBJ and the sCMOS camera suchthat the dichromatic beam splitter DBS separates fluorescent light fromthe light collected by the objective OBJ and the knife-edge mirror KEMsubsequently combines the fluorescent light and the remainder of thelight collected by the objective OBJ prior to their simultaneous imagingby the sCMOS camera. In this manner the monitoring device 36 isconfigured to detect light that is created by the illumination of eachparticle by the guided light and that leaves the channel 32 by passingthrough the channel wall.

The imaged area may be immersed in index-matching oil to overcomeaberrations caused by the outer cylindrical shape of the optical fibre38. Alternatively the optical fibre's cladding may be index-matched to aflat glass slide to obtain an almost isotropic point-like imaging of theparticles on the sCMOS camera.

Use of the particle detection apparatus 30 to detect one or moreparticles is described as follows.

Initially the particles to be conveyed along the channel path islabelled with fluorophores. After the particles to be conveyed along thechannel path are introduced into the channel 32, the particles freelydiffuse in a fluid (e.g. liquid or gas) present in the channel path.

During the conveyance of the particles along the channel path, light isintroduced into the channel 32 via the inlet to illuminate each particlelocated in the channel path. Due to the confinement of the guided lightin the channel 32 and the sub-wavelength dimension of the bore of thechannel 32, the illumination of each particle results in scattering dueto the polarizability and size of the particles and fluorescence due tothe presence of the fluorophores. When the size of a particle is smallerthan the wavelength of the guided light, illumination of that particleresults in coherent and/or incoherent light scattering.

Part of the resultant scattered light and fluorescent light leave thechannel 32 through the channel wall at a non-zero angle to a guideddirection of the guided light. The objective collects the scatteredlight and fluorescent light, which is then transmitted to the sCMOScamera for imaging. The sCMOS camera subsequently processes the detectedlight so as to create an output image of each illuminated particle tothereby permit visualisation of each illuminated particle.

Meanwhile any light that is not scattered by the particles stays guidedalong the channel path. This results in excellent signal to backgroundand signal to noise ratios and thereby enhances the detection of eachilluminated particle, thus preventing the detected scattered light frombeing overwhelmed by direct detection of the residual scattering of theilluminating guided light.

Detection of the scattered light and fluorescent light by the particledetection apparatus 30 permits study of each particle in different ways,examples of which are as follows.

The configuration of the particle detection apparatus 30 permit use ofthe effects of coherent and/or incoherent light scattering to detectvery small particles, especially those in the sub-100 nm range.

FIGS. 3 to 5 illustrate the scattering intensity as a function ofposition when an aqueous suspension of dielectric latex nanoparticlesare conveyed along the channel path. The dielectric latex nanoparticleshave nominal diameters of 19 nm, 35 nm and 51 nm, each of whichrespectively corresponds to scattering cross-sections of 0.0023 nm²,0.09 nm² and 0.86 nm² for a wavelength of 670 nm.

FIG. 3 is an exemplary raw image of the latex nanoparticles with anexposure time of 1 ms, while FIG. 4 depicts the same image inlogarithmic false colour. FIG. 5 illustrates a semi-logarithmic plot ofthe sum scattering intensity as a function of position. It can be seenfrom FIGS. 3 and 5 that the particle detection apparatus 30 is capableof detecting the dielectric latex nanoparticles with nominal diametersof 19 nm, 35 nm and 51 nm. In addition the particle detection apparatus30 enables measurement of the scattering intensity and tracking of eachparticle's motion through detection of the scattered light.

Such measurement of the scattering intensity of each detected particleand such tracking of each particle's motion not only providesinformation about each detected particle, but also permits the study ofthe thermal diffusion and thereby the hydrodynamic behaviour of eachparticle.

FIG. 6 illustrates the tracking of the positions of single cowpeachlorotic mottle viruses (CCMV) with time using the particle detectionapparatus 30. By using the particle detection apparatus 30 to track theposition of each detected particle with time, their Brownian motion maybe analysed to yield the diffusion constant of each detected particleand hence its size via the Einstein-Stokes equation, as later discussedin this specification. It can be seen from FIG. 6 that the particledetection apparatus 30 is capable of tracking single CCMW with sizes inthe range of 20 nm.

FIG. 7 illustrates the tracking of the positions of the above dielectriclatex nanoparticles and 26 nm single cowpea chlorotic mottle viruses(CCMV) with time using the particle detection apparatus 30.

FIG. 7 includes a plot of the average detected scattering intensity as afunction of the extracted diffusion constants for the above dielectriclatex particles and 26 nm single CCMV, and a histogram of the logarithmof the detected scattering intensities for the tracked particles. It canbe seen from the plot and histogram of FIG. 7, as indicated by referencenumerals 42,44,46,50,52,54, that the different dielectric latexnanoparticles with different nominal diameters exhibits significantlydifferent scattering intensities from each other. The particle detectionis therefore capable of distinguishing between the different dielectriclatex nanoparticles with different nominal diameters based on theirdifferent scattering cross-sections.

Moreover it can be seem from the histogram of FIG. 7 that the singleCCMV due to their lower index contrast exhibit a lower scatteringintensity 56 than that 50,52,54 of the dielectric latex nanoparticles,and so the particle detection apparatus 30 is capable of distinguishingbetween the dielectric latex nanoparticles and the single CCMV.

Furthermore measurement of the scattering intensity and theEinstein-Stokes diffusion constant of each detected particle allows anaggregate of particles to be distinguished from a single larger particleeven if they both exhibit similar scattering intensities.

Each detected particle's diffusion constant and size is obtained by:

-   -   obtaining a displacement histogram for each time interval;    -   verifying that the displacement histogram is Gaussian;    -   calculate a corresponding mean square displacement (MSD) using        the variance of the displacement histogram;    -   calculating the diffusion constant as half of the fit to the        slope of the MSD against the time interval for small intervals;    -   calculating the hydrodynamic diameter of each detected particle        using the Einstein-Stokes equation.

The Einstein-Stokes equation for water at room temperature is:

${{Hydrodynamic}\mspace{14mu}{diameter}\mspace{11mu}({\mu m})} = \frac{4.11}{6 \cdot \pi \cdot D}$where D is the diffusion constant.

FIG. 8 illustrates a theoretical comparison of scattering cross-sectionversus diffusion constant for spherical particles of differentrefractive indices and for particle aggregates.

The circular dots 58 in FIG. 8 represent a theoretical model of thescattering cross-section versus diffusion constant of aggregates ofmultiple 20 nm latex nanoparticles. The upper straight line 60represents a theoretical model of scattering cross-section versusdiffusion constant for single full spherical latex particles ofdifferent sizes and a refractive index of 1.65. The lower straight line62 represents a theoretical model of scattering cross-section versusdiffusion constant for single full spherical protein particles ofdifferent sizes and a refractive index of 1.4.

It can be seen from FIG. 8 that the scattering intensity from aggregatesof multiple 20 nm latex nanoparticles exhibits a scaling behaviour withdiffusion constant that is different from the scaling behaviours of thefull spherical particles of different sizes. Thus, this difference inscaling behaviours allows an aggregate of particles to be distinguishedfrom a single larger particle even if they both exhibit similarscattering intensities.

The configuration of the monitoring device 36 to detect scattered lightpermits the use of metallic, semiconductor or organic contrast agents toenhance the polarizability of each particle and thereby enhance thedetection sensitivity of the particle detection apparatus 30.

FIG. 9 illustrates the tracking of 20 nm to 50 nm latex particles usingthe particle detection apparatus 30. It can be seen from FIG. 9 that thescattering intensity 64 is brighter than the fluorescence intensity 66.By using the particle detection apparatus 30 to track the position ofeach detected particle with time, it is evident from FIG. 10 that thescattering intensity remains unchanged while FIG. 11 shows that thefluorescence intensity diminishes with time due to stepwisephoto-bleaching. Thus, the use of the particle detection apparatus 30 todetect both scattered and fluorescent light permits simultaneousmeasurement of the scattering and fluorescence in order to utilise theresultant stepwise photo-bleaching to count the number of fluorophores.

In addition the configuration of the monitoring device 36 to detect bothscattered and fluorescent light permits simultaneous measurement of thescattering and fluorescence in order to measure an emission spectrum ofeach particle. This permits identification of each particle on the basisof its spectral features.

Measurement of the scattering intensity of the scattered light permitsthe study of particle interaction. For example, measurement of thescattering intensity of the scattered light permits the study ofparticle binding and unbinding events through detection of quadraticchanges in the scattering intensity, the spectral response or thediffusion constant of each particle.

Optionally the monitoring device 36 may be configured to measure thespectrum of the scattered light and/or the distribution of the scatteredlight over a plurality of directions.

The thermal diffusion of small particles in a liquid is inverselyproportional to its size and can reach tens of square micrometers persecond for a 10-nanometer spherical particle in water, thus limiting theavailable detection period to the duration in which the particle spendsin an imaging focal place.

On the other hand the configuration of the particle detection apparatus30 of FIG. 1 allows the particles in the channel path to stayilluminated by the guided light and thereby remain in the imaging planeand not diffuse out of focus. Keeping the particles in the illuminationplane of the guide light not only obviates the need for immobilisationof particles in a restricted volume, such as that performed in cryogenicelectron microscopy, and thereby results in a less complex and cheaperparticle detection apparatus 30, but also provides a prolonged detectionperiod that permits enhanced real-time tracking of each particle andincreases the obtainable amount of information about each particle.

In contrast to scattered light detection, in fluorescent microscopy, thespeed is limited by the fluorescence emission rate, and the availabledetection period is truncated by photo-bleaching of the fluorescentlight.

The improved detection capabilities of the particle detection apparatus30 of the invention as set out above not only obviates the need for aspecialised monitoring device 36 to detect each particle and therebypermits use of simpler and cheaper monitoring devices, such as anoptical microscope, various kinds of photo-detectors, line CCD detectorsor a smartphone camera, but also permits detection of each particleunder ambient conditions, instead of specific conditions as required bycryogenic electron microscopy.

It is envisaged that, in other embodiments of the invention, theparticle detection apparatus 30 may include a driving mechanism fordriving each particle to flow along the channel path. In one example ofsuch a driving mechanism, electrodes may be incorporated into thechannel 32 to permit use of an electrophoretic force to steer eachparticle along the channel path.

In another example of such a driving mechanism, the driving mechanismmay be external to the channel 32 and may be configured to generate anelectric field so as to provide an electrophoretic force to steer eachparticle along the channel path. Such a driving mechanism may be in theform of a capillary electrophoretic setup. This thereby permits thecombination of capillary electrophoresis with the use of the particledetection apparatus 30 to track the motion of each particle.

FIG. 17 shows (a) the scattered intensity of three individual 60 nm goldnanoparticles that are electrophoretically actuated inside the opticalfibre 38 when measured under the same settings of FIG. 2, (b) thefalse-colour images of the same three individual 60 nm goldnanoparticles that are electrophoretically actuated inside the opticalfibre 38 when measured under the same settings of FIG. 2, and (c) themanipulation of the position of these nanoparticles using an externalspatially-uniform temporally alternating electric field.

The zig-zag tracks in FIG. 17(c) depict the position of these particlesas a function of time recorded at 1 kHz. This result clearly shows thatthe capability of the particle detection apparatus 30 to track themotion of the gold nanoparticles beneficially provides active feedbackthat permits the remote manipulation of the gold nanoparticles in amanner that is strong and fast enough to overcome their thermal Brownianmotion.

The combination of capillary electrophoresis with the use of theparticle detection apparatus 30 to track the motion of each particle maybe, for example, applied to the study of the charging dynamics ofproteins at a single-particle level in biologically relevantenvironments.

In other embodiments of the invention, it is envisaged that the particledetection apparatus may include:

-   -   a plurality of channels, each channel including an inlet and at        least one channel wall, each inlet permitting light to be        introduced into the respective channel, the or each channel wall        being arranged to define a respective channel path through which        light may propagate;    -   a plurality of light sources, the plurality of light sources        being configured to introduce light into the channel via the        inlet; and/or    -   a plurality of monitoring devices, the plurality of monitoring        devices being configured to detect scattered light that is        created by the illumination of the or each particle by the        guided light and that leaves the channel by passing through the        or each channel wall.

When the particle detection apparatus includes a plurality of channels,the monitoring device may be configured to simultaneously detectscattered light that is created by the illumination of the or eachparticle by the guided light and that leaves each channel by passingthrough the or each corresponding channel wall.

When the particle detection apparatus includes a plurality of lightsources, the plurality of light sources may be configured tosimultaneously introduce light into the channel via the inlet.

When the particle detection apparatus includes a plurality of monitoringdevices, the plurality of monitoring devices may be configured tosimultaneously detect scattered light that is created by theillumination of the or each particle by the guided light and that leavesthe channel by passing through the or each channel wall.

Optionally the channel may be formed in an exemplary measuring cell,details of which are set out as follows.

The measuring cell is configured as an optical waveguide for guiding alight beam, said waveguide comprising a core having a refractive indexn_(k), which extends along a longitudinal axis of said waveguide, has across-sectional area A_(K) of less than 80 μm² in a cross sectionperpendicular to the longitudinal axis, and which is surrounded by acladding having a smaller refractive index than n_(K), wherein saidcavity forms a channel, extending along the longitudinal axis, beingformed inside of or in contact with said core, and having at least oneopen end with an opening area A_(H) of less than 0.2 μm².

The channel is formed in an optical fibre, e.g. in a step-index orgradient-index fibre or in another waveguide structure, for instance ina semiconductor microchip manufactured by etching and depositionprocesses. The light conduction of the optical waveguide is achieved byway of different refractive indices of core and cladding.

The hollow channel serves to receive a fluid medium which contains thesample particles to be analyzed. The fluid medium is here enclosed inthe hollow channel or it is guided through the hollow channel by flowingtherethrough. The sample particles contained therein can move along thelongitudinal axis of the hollow channel, but they are limited in theirmotion in the directions perpendicular thereto by the width dimension ofthe channel. In this respect the width dimension of the channelspatially limits the path of movement of the sample particles in lateraldirection to a certain degree.

For this purpose the hollow channel is restricted to a width which isdefined by an opening area of less than 0.2 μm². In a channel with acircular cross-section this corresponds to a diameter of less than 500nm. The depth of focus of simple optical microscopes is enough fordetecting sample particles within that range. Optionally, however, thehollow channel is even smaller; it has e.g. a diameter in the range of20 nm to 500 nm, optionally in the range of 50 nm to 300 nm if it iscircular in a cross section perpendicular to the longitudinal axis.

The spatial inclusion of the light for an excitation radiation coupledat the front side into the core is all the more pronounced and the lightintensity guided in the core is the higher the smaller thecross-sectional area of the core is. A small cross-sectional area of thecore facilitates the implementation of a single-mode light conductionalso in the case of a shortwave excitation radiation and in the case ofa great refractive-index difference between core and cladding. Thishelps to increase the radiation energy penetrating into the hollowchannel and thereby to improve the illumination of the channel. For thispurpose the core has a cross-sectional area A_(K) of less than 80 μm² ina cross section perpendicular to the longitudinal axis. In a core havinga circular cross-section this corresponds to a diameter of less than 10μm. Optionally, however, the core diameter is even smaller; forinstance, it has a diameter less than 3 μm if it is circular in a crosssection perpendicular to the longitudinal axis. A core diameter of lessthan 1 μm is not preferred from a practical point of view.

The channel extends within the optical waveguide along the longitudinalaxis inside of or in contact with the core. It shares a contact surfacewith the core. Viewed in a cross section perpendicular to thelongitudinal axis (for the sake of simplicity, also briefly called“radial cross-section” hereinafter without the intention to restrict thecross section to the circular shape), the channel extends eitherdirectly next to or in contact with the core, or it extends partly or,optionally, fully within the core. At any rate, the hollow channel isdefined at least partly, optionally completely, by core material.

The excitation radiation is guided via the core/cladding structure ofthe optical waveguide along the longitudinal axis and in the hollowchannel along a measurement section. Under ray-optical aspects the lightconduction is based on total reflection on the condition n_(K)>n_(M)(refractive indices at the wavelength of the D line of the sodium vaporlamp). The light guided in the core can here penetrate into the channeland “illuminate” the hollow channel. This light intensity transmissionfrom the core into the hollow channel is not limited to points orlocations, but takes place over quite a long section, e.g. along thewhole contact surface between core and channel. The light that haspenetrated into the channel can thus serve as radiation for excitingscattering or other states of the sample particles existing within thechannel, namely over a rather large section, which also permits themonitoring of the movement of the sample particles over a rather longsection. The restricted opening width of the hollow channel preventssample particles from migrating out of the excitation light field.

It is advantageous for an efficient illumination of the sample particleswhen the intensity distribution within the channel is as great aspossible and is homogeneous both in radial and in axial direction. Theproportion of the radiation intensity arriving at the channel can serveas a measure of the suitability of the measuring cell design. In thisrespect the ratio of the intensity minimum in the channel and themaximum intensity in the core is regarded as a measured value. Thismeasured value should be at least 1%, optionally it is 30% or more.

As is known, the number of the light propagation modes in a step-indextype optical waveguide depends for a given wavelength substantially onthe refractive-index difference between core and cladding and on thecore diameter. With respect to a reproducible transfer of guidedexcitation light into the hollow channel, preference is given to themeasuring cell in which the difference between the refractive indices ofthe core and the cladding, the cross-sectional area of the core and thewavelength of the guided light beam are coordinated such that thefundamental mode of the light beam and not more than 20 further modesmay be propagated.

In the case of multispectral excitation radiation, it is advantageouswhen this condition for the light conduction is fulfilled for theshortest wavelength of the spectrum.

Particularly preferred is per se the measuring cell in which only onesingle mode, the fundamental mode, is formed, as is the case withso-called single-mode fibres. Here, the guided light intensity is solelytransmitted by the fundamental mode, which facilitates the transfer of alight intensity as high as possible into the channel. With theconfiguration of several modes, the light intensity is distributed overthese modes, which on the one hand leads to a low intensity maximum inthe core and in the hollow channel. On the other hand, the energydistribution over individual modes is difficult to determine, so that inthe case of a multimodal excitation radiation the real intensitydistribution in the cavity can be defined less accurately than in thecase of a single-mode radiation, which makes the evaluation of thescattered radiation more difficult. The smaller the core (the corediameter), the less light modes are possible under otherwise identicalconditions. Therefore, it is true that the single modedness of the lighttransmission can be ensured in principle in that the size of the core(the core diameter) is set to be sufficiently small. However, a smallcore size also entails increased manufacturing and adjusting efforts.The smaller the core, the more complicated is the transmission of lightinto said core. In practice, it is moreover difficult to exactly selectthe predetermined core diameter and to maintain it over the whole lengthof the measuring cell. Therefore, on the other hand, a core diameterthat is as large as possible would be optimal, in the case of which thesingle modedness of the light transmission is just barely ensured.Moreover, the channel changes the boundary conditions (boundaryconditions regarding the Maxwell equations) underlying the creation ofthe light modes, so that especially in the case of a near cut-off designwith respect to the single-mode light propagation (near the so-calledcut-off wavelength) even higher modes may easily be formed. Therefore,apart from the fundamental mode, a certain number of higher modes isconsidered to be acceptable as long as this number does not exceed 20modes.

A measuring cell in which core and cladding are composed of highlysiliceous glass has turned out to be useful.

“Highly siliceous glass” stands for an optically transparent glasshaving a SiO₂ content that is at least 60% by wt.

In this connection preference is given to a measuring cell in which thecore consists of quartz glass which is doped with germanium oxide, andthat the cladding consists of quartz glass, which is not doped or whichis doped with a component—in particular with fluorine —capable ofdecreasing the refractive index of quartz glass.

Quartz glass is substantially transparent over a wide wavelength rangebetween about 150 nm and 3000 nm. Hence, the measuring cell allows anexcitation radiation with wavelengths in the range from UV to infrared,with a small scattering contribution by the walls of the measuring cellitself. Moreover, the material quartz glass helps to implement hollowchannels of a particularly small opening cross-section of for instanceless than 100 nm owing to a relatively great temperature interval inwhich hot formation can be carried out.

Germanium oxide brings about an increase in the refractive index ofquartz glass. It has been found that the light intensity guided in thecore and thus also the intensity penetrating into the hollow channel isthe higher, the greater the refractive index difference between core andcladding is. When the core is doped with germanium oxide and thecladding is simultaneously doped with fluorine, a particularly greatrefractive-index difference can be established at the core/claddingboundary. This difference is optionally at least 8×10⁻³.

Doping the core glass with germanium oxide has a drawback in thatgermanium may evaporate during high temperature process steps, therebychanging the radial profile of the refractive index. Therefore, the coremay consist of undoped quartz glass, and the cladding consists of quartzglass having a refractive index n_(c), said quartz glass is doped with acomponent—in particular with fluorine—capable of decreasing therefractive index of quartz glass.

Undoped quartz glass has high optical transmittance and a viscosityhigher than doped quartz glass. A high viscosity of the core glassfacilitates maintenance of even very small channels inside the coreregion compared to core glasses with lower viscosity.

It has been found that both a small core diameter and a large differencein refractive contribute to a high intensity of radiation penetratingfrom core into the channel. In view of that it is advantageous if thecore glass is made of undoped quartz glass, and the differencen_(k)−n_(c) is at least 16×10⁻³, optionally at least 20×10⁻³.

It has turned out to be useful when the core and the cladding are madeof massive, solid material.

Both core and cladding consist of solid and massive bulk material. Bothcore and cladding exhibit a nominally homogeneous refractive-indexprofile in radial cross-section. Local changes in the refractive indexdue to high temperatures and diffusion processes during themanufacturing process can hardly be prevented. The cladding, however, iswithout an internal boundary, such as e.g. a further core or a furtherchannel. Likewise, apart from the contact area with the single hollowchannel and with the single cladding, the core has no furtherboundaries. In the absence of boundaries the measuring cell issubstantially free of boundary-related scattering; specifically, it hasa particularly scatter-free cladding.

Moreover, the effect of so-called mode coupling is avoided, by whichenergy of a mode is coupled into another mode. This effect may occurwhen different light-guiding regions are present. Mode coupling has theeffect that the energy of the light is periodically exchanged betweenthe different light-guiding regions. This, however, has the effect thatthe scattering rate along the longitudinal axis of the optical waveguideis varying. Thus, the sample particle would scatter to different degreesaxially, depending on the various positions of the fibre, as the lightfield is periodically varying. This effect could possibly also occur ifthe hollow channel is not formed inside the core, but away from it. Sucha mode coupling may be entirely excluded, so that the intensity of thelight (with the exception of the (negligible) attenuation) is axiallyindependent.

It has also turned out to be advantageous when in a cross-sectionperpendicular to the longitudinal axis, the core is circular having adiameter of less than 10 μm and a core center point which is locatedinside a respective cross-sectional area of the hollow channel. Thehollow channel is optionally provided at a position at which theconditions for the penetration of light out of the core are optimal.Ideally, this position is located in the core center point. The channel,however, can also extend laterally therefrom. In the simplest case ofthe measuring cell, the core, cladding and hollow channel extendcoaxially relative to one another in the optical waveguide. Hollowchannel and core are here circular in radial cross-section andconcentric relative to each other. The rotation symmetry of themeasuring cell is of advantage during use insofar as the measuringconditions and measuring results are independent of the spatialorientation thereof inside the measuring equipment. The core diameter isoptionally small and is less than 10 μm, optionally less than 3 μm. Theadvantages of a small core diameter have been explained further above inconnection with its small radial cross-section.

It is advantageous when, on the one hand, high radiation energy isguided in the core, which radiation energy, on the other hand, canpenetrate into the channel as efficiently as possible. In this respectpreference is given to the measuring cell in which the channel extendsentirely inside the core, wherein in a cross section perpendicular tothe longitudinal axis, the core has a cross-sectional area A_(K) and thechannel has a cross-sectional area A_(H), wherein the ratio A_(K)/A_(H)is greater than 4, optionally greater than 20.

In this case the hollow channel viewed in radial cross-section extendsentirely within the core. It is surrounded over its length by corematerial, so that the radiation energy guided in the core canefficiently penetrate into the hollow channel. In this case, however,the opening area of the hollow channel (viewed in radial cross-section)is completely at the expense of the cross-sectional area of the core. Tobe able to provide a sufficiently high radiation energy inside the core,the lateral dimensions of the channel (e.g. its inner diameter) haveoptionally to be adjusted such that the remaining cross-sectional areaof the core is still greater at least by the factor 4, optionally by thefactor 20, than the opening area of the hollow channel.

The optical waveguide is here optionally configured as a step-indexfibre with a channel, wherein the channel has an opening width which issmaller than the wavelength of a light beam to be guided in the opticalwaveguide.

In an optional configuration of the measuring cell, the opticalwaveguide is configured as an optical fibre with a circularcross-section, wherein the cladding has an outer diameter in the rangeof 150 μm to 300 μm.

A fibre of this thickness is on the one hand still flexible and thusless prone to fracture than a rigid fibre of a greater thickness. On theother hand, its thickness is greater than the optical single-modestandard fibres, so that it can be handled more easily. The cladding canadditionally be provided with a protective covering.

The essential advantages of the measuring cell are:

-   1. spatial inclusion of the sample material;-   2. low background signal;-   3. possibility of measuring very small particles/molecules; and-   4. easy integration of the measuring equipment into existing,    commercially available and wide-spread measuring instruments and    thus small acquisition costs together with moderate production costs    of the optical waveguide.

The measuring cell is obtained from the preform by elongation, whereinthe measuring cell is present in the form of an optical fibre withlight-conducting hollow channel. The hollow channel extends in radialcross-section entirely within the core of the optical fibre. It issurrounded over its length by core material, resulting in an effectivepenetration of radiation energy into the channel, with the radiationenergy being guided in the core. To be able to provide a sufficientlygreat amount of radiation energy in the core, the lateral dimensions ofthe channel (e.g. its inner diameter) must optionally be set such thatthe remaining cross-sectional area of the core is still by at least thefactor 4, optionally by the factor 20, greater than the cross-sectionalarea of the channel.

In the simplest case, this ratio in the preform from which the opticalfibre is drawn true to scale is already predetermined.

Details of a further exemplary measuring cell is set out as follows,with reference to FIGS. 12 to 16.

The exemplary measuring cell is used in the form of an optical hollowfibre 1 with a core 2, a cladding 3, and a light-guiding channel 4.

Due to the core/cladding structure of the fibre an introduced laserlight is guided in the fibre core 2 and reaches—also within the channel4—an intensity that is sufficient for optically analyzing sample volumesintroduced into this cavity. This is the case whenever the width of thehollow channel 4 is in the order of magnitude of the wavelength of theguided light or is smaller. Thus, the light conduction of the hollowfibre 1 makes it possible to illuminate the hollow channel volume overthe whole length in a quasi-uniform manner. As a result, the region fromwhich light can be detected for the microscopic analysis is not limitedto a spot region.

FIG. 12 schematically shows the measuring cell in the form of an opticalfibre 1 with the light-guiding hollow channel 4 in a top view on thefront side of the fibre. The core 2 consists of germanium oxide-dopedquartz glass and has an outer diameter of 3 μm. The cladding 3 adjoiningthe core 2 consists of undoped quartz glass and has an outer diameter of200 μm. The channel 4 has a diameter of 200 nm. The difference betweenthe refractive indices of the quartz glasses of core 2 and cladding 3 is0.008. The channel 4, the core 2, and the cladding 3 extend coaxiallyabout the longitudinal axis 9 and are concentric to one another in theplane of representation of FIG. 12. Apart from the hollow channel 4,neither the core 2 nor the cladding 3 exhibit other structuralirregularities or inhomogeneities that might lead to scattering. A fewcentimeters of the fibre length are sufficient for a respectivemeasurement.

Hence, the optical fibre 1 consists of quartz glass and, in comparisonwith other optical materials, such as multicomponent glasses or opticalplastics, it exhibits low attenuation for light from the ultraviolet upand far into the infrared wavelength range and thus also exhibits anexcellent light scattering. This property reduces the scatteringbackground in the measurement to a minimum and allows a goodsignal-to-noise ratio. This is particularly important in the case ofvery small sample particles because the scattering signal of the sampleparticles correlates in an over-linearly reciprocal manner with theparticle diameter. The described reduction of the scattering backgroundis therefore positively noticed particularly in the analysis ofespecially small sample particles 5, as are e.g. found in biologicalprocesses and which could so far not be analyzed with this methodbecause of their small dimension. Viruses should here be mentioned byway of example.

For the same reason it is moreover possible in many cases to dispensewith the additional marking of the sample particles to be analyzed withfluorescent substances. The amount of the intensity of the scatteredlight is not physically limited in contrast to the saturation behaviorof the fluorescence of every fluorescent molecule, but dependsparticularly on the local intensity of the excitation light. When theexcitation intensity can be increased, the whole intensity of thescattered light is increasing. Sufficiently high scattering results canthereby also be achieved within short time intervals. This makes itpossible to directly track biochemical processes, including possibleintermediate steps, and thereby to measure the properties which can beanalyzed with this method.

FIG. 13 is a micrograph of a fracture surface in an intermediate productfrom which the optical fibre is obtained with core 2, cladding 3 and thelight-guiding hollow channel 4 by true-to-scale elongation after afurther production step of increasing the amount of cladding material.The optical fibre 1 with the hollow channel 4 has to fulfill one or moreof the following tasks:

-   -   reception of the fluid with one or more sample particles 5.    -   restriction of the space in which the sample particles 5 can        move. The sample particles 5 are here substantially restricted        to a one-dimensional movement in the direction of the        longitudinal axis 9.    -   transmission of the excitation light to the sample particles 5.        It is the aim to achieve a light intensity which is as high as        possible within the whole channel 4, if possible.    -   especially in the case where elastic light scattering is to be        detected, the minimization of the background scattering level is        important (by use of quartz glass in comparison with other        materials for the optical fibre).

The constructional design of the optical fibre 1 is chosen such that thelight intensity within the hollow channel 4 is as high as possible. Forexample,

-   -   the optical fibre comprises at least a light-guiding fibre core        2, a cladding 3, and a hollow channel 4;    -   the hollow channel 4 is located in or directly on the fibre core        2, so that a proportion of the excitation light, which is        supplied through the fibre core 2, penetrates into the hollow        channel 4. Optionally, the hollow channel 4 is completely        positioned with the fibre core 2;    -   the optical fibre 1 is a single-mode fibre or a fibre with the        fundamental mode and otherwise with just a small number of modes        (optionally with the fundamental mode and less than 20 secondary        modes);    -   the hollow channel 4 is a cavity which is open at both sides or        is closed (in the last-mentioned case, the sample medium is        enclosed, for instance in that the ends of the hollow fibre are        spliced with other optical fibres without hollow channel);    -   the hollow channel 5, viewed in radial cross-section, has a        circular shape and a diameter which is in the order of magnitude        of the wavelength of the guided light or less;    -   the optical fibre 1 has a high numerical aperture and thus a        core diameter which is as small as possible, so that the light        intensity in the hollow channel 4 is maximized; and    -   the optical fibre 1 consists of doped and/or undoped quartz        glass to prevent a strong scattering background.

FIG. 14 shows results of a simulation regarding the radial intensityprofile within the optical fibre 1. On the ordinate of the diagram, thez-component (along the longitudinal fibre axis 9) of the Poynting vector“l” is plotted (in relative units) against the radial position “p” (innm) starting from the centre of the hollow channel K (p=0). The amountof the Poynting vector corresponds to the intensity of the fundamentalmode guided in the fibre in the event that the hollow channel 4 isfilled with water (refractive index of water: 1.33).

Curves A500 and A1000 represent the radial intensity profile of a fibre,whereby A500 is simulated for a guided light having a wavelength of 500nm, and A1000 is simulated for a guided light having a wavelength of1000 nm. In this case, the refractive index difference between cladding2 and core 3 is 0.008 (typical order of magnitude of standardsingle-mode fibres); the core has a diameter of 3 μm.

In comparison with an undisturbed core, one obtains a differentintensity curve of the fundamental mode in a core 3 having a centralbore 4 (which is 200 nm in this case). The intensity maximum is notlocated in the fibre center, but approximately in the center betweeninner wall of the cladding and wall of the hollow channel. It can beseen that within the water-filled hollow channel K the intensity dropsonly slightly and is even at the minimum in the same order of magnitudeof the amplitude of the total curve.

The ratio of the intensity minimum in the hollow channel K (in thecentre) and the maximum intensity in the core is about 50% in thisparticular design.

By comparison, curves B500 and B1000 show the intensity distribution ofthe fundamental mode in a fibre with an increased refractive indexdifference between core and cladding. The core consists of undopedquartz glass and has an outer diameter of 1.7 μm. The cladding consistsof quartz glass which is doped with fluorine and has an outer diameterof 200 μm. The hollow channel has a diameter of 200 nm. The differencebetween the refractive indices of the quartz glasses of core andcladding is here 0.025. Curve B500 is simulated for a guided lighthaving a wavelength of 500 nm. Curve B1000 is simulated for a guidedlight having a wavelength of 1000 nm.

Particularly on account of the higher refractive index differencecompared to curves A500 and A1000, for this measuring cell the totalradiation intensity guided in the hollow channel of the total intensityof the radiation inside the core 3 is larger than in the respective B500and B1000 curves. In case of curve B500, the ratio of the intensityminimum in the hollow channel 4 (in the centre) and the maximumintensity in the core 3 is about 60%.

The comparison between curves A500 and A1000 and curves B500 and B1000shows that the guided light with higher wavelength (1000 nm) results ina more even radial distribution profile of intensity I in the coreregion as well as in the channel region K.

The diagram of FIG. 15 illustrates the influence of the inner diameterof the hollow channel on the radiation intensity guided within thehollow channel for two particular wavelengths 500 nm and 1000 nm. As isFIG. 14, curves A500 and A1000 represent the radial intensity profile ofa fiber, whereby curve A500 is simulated for a guided light having awavelength of 500 nm, A1000 is simulated for a guided light having awavelength of 1000 nm while the refractive index difference between core3 and cladding 2 is 0.008 and the diameter of the core is 3.0 μm inaccordance with curves A500 and A1000 from FIG. 14. Curve B500 issimulated for a guided light having a wavelength of 500 nm, B1000 issimulated for a guided light having a wavelength of 1000 nm each for thecase of a refractive index difference of 0.025 and a core diameter of1.7 μm. On the ordinate of the diagram, the ratio l_(min)/l_(max) (in %)of the minimum of the z-component of the Poynting vector inside thechannel 4 and the maximum of the Poynting vector “l” in the core isplotted against the diameter d (in nm; as opening width) of the hole.Here, this ratio represents the amount of decrease of light intensitywithin the channel.

It is evident from this that the radiation intensity guided in thehollow channel depends on the inner diameter of the hollow channel. Thesmaller the diameter, the higher the intensity values at the minimainside the hole will become. On the other hand, the smaller the hole,the more difficult it will become to manufacture as well as to work withthe device (evidentially also only particles smaller than the hole sizeare physically capable of entering it). Consequently, a trade-offbetween required minimal intensity and minimal hole diameter yields apractically favourable hole diameter of 50 nm to 300 nm in diameter.

The optical fiber 1 with hollow channel 4 is drawn from a preform. Theproduction of a preform for a measuring cell with a refractive indexdifference of the fiber represented by curve A in FIG. 14 shall beexplained hereinafter with reference to an example and with reference toFIG. 16 in more detail.

In a first method step, a so-called substrate tube 81 is provided. Thesubstrate tube 81 consists of undoped quartz glass and has an innerdiameter of 21 mm and a wall thickness of 2 mm. On the inner wall of thebore of the substrate tube 81, a core layer 82 of germanium-containingquartz glass is deposited on the inside of the substrate tube. Theundoped quartz glass will subsequently serve as the cladding materialaccording to the known MCVD method. The germanium content of the corelayer 82 is set such as to meet a refractive index difference of 0.008with respect to the undoped quartz glass of the cladding material. Thesubstrate tube 81 which is thereby coated on the inside is subsequentlycollapsed to form a quartz glass tube 84, wherein a bore 85 with adiameter of 0.5 mm is maintained. The germanium-containing layer forms ahollow core 86 with an outer diameter of about 3 mm. The outer wall ofthe quartz glass tube 84 is flame-polished by means of an oxyhydrogenburner. The quartz glass tube 84 cleaned in this way is elongated in adrawing process without any tools to a thin tube 89 having an outerdiameter of roughly 2 mm. During the elongation process the inner holeof the quartz glass tube and of the drawn-off tube strand, respectively,is flushed with nitrogen. The inner hole 87 of the thin quartz glasstube 89 obtained thereby has a diameter of about just below 100 μm. Thequartz glass tube 89 is overcladded in a further method step with aso-called jacket tube 91 of undoped quartz glass. The thin quartz glasstube 89 is introduced into the bore of the jacket tube, it is coaxiallycentred therein and fused therewith zone by zone to form a thick-walledtubular preform 90. The tubular preform 90 produced thereby has an outerdiameter of roughly 30 mm and in a radial cross-section it shows aconcentric arrangement of inner hole 87.

The preform 90 has a single-mode step-index design and a coaxial centralbore, in addition. It is drawn into an optical fiber 100 with alight-guiding hollow channel 101. In order to avoid total collapsing,the inner hole 87 is pressurized with nitrogen during the drawingprocess. The resulting fiber 100 has a nominal diameter of 200 μm. It isformed from a coaxial arrangement of an inner hole 101, a core region103 of Ge-doped quartz glass and an outer cladding region 102 of undopedquartz glass. The inner hole 101 has a diameter of just below 600 nm,the core region 103 has an outer diameter of just below 3 μm.

Hereinafter, an alternative manufacturing process of a measuring cellshall be described. The manufacturing process involves a deposition stepin which a layer of fluorine-doped quartz glass is produced on a supporttube by means of a standard POD method (Plasma-assisted OutsideDeposition). The support tube consists of undoped synthetic quartzglass. It has an inner diameter of 5 mm and an outer diameter of 40 mm.To this end SiCl₄, oxygen and SF₆ are supplied to a plasma burner andare converter into SiO₂ particles in a burner flame assigned to theplasma burner. Since the plasma burner is reversingly moved along thesupport tube from one end to the other one, the SiO₂ particles aredeposited in layers on the outer cylindrical surface of the support tuberotating about its longitudinal axis. It is thereby possible toincorporate high fluorine concentrations of more than 5 wt.-% in thequartz glass network of the fluorine doped quartz glass layer having athickness of 15 mm.

Following the deposition process a heated etching gas stream of SF₆ isintroduced into the centre bore of the support tube. The etching gasstream of SF₆ is configured such that the support tube is not completelyremoved, but a layer of undoped silica with a thickness of 15 mmremains. A mechanical treatment of the inner bore of the tubular form(=starter tube) is not needed.

The starter tube produced in this way is subsequently drawn in anelongation process without any tools into a double-walled tube having acore layer of undoped silica and a cladding layer of fluorine-dopedquartz glass. To this end an internal pressure which in comparison withthe externally applied external pressure is raised by 5 mbar ismaintained in the inner bore. This yields a double-walled tube whichcomprises an inner wall which is smoothed by hot deformation and has aparticularly high surface quality and an exact width of the inner boreover the whole length of the tube.

The resulting double-walled tube is further processed in a second PODdeposition process for the further deposition of a layer offluorine-doped quartz glass, as has been described above for the makingof the starter tube, resulting in a thick-walled “mother tube”.

The mother tube is elongated in order to obtain an optical fiber with alight-guiding hollow channel, as has been explained above with referenceto curve B in FIG. 14. Besides the coaxial inner hollow channel theresulting fiber has single-mode step-index design. It has a core ofundoped silica and a cladding of fluorine-doped silica. Segments withthe desired lengths are produced from the optical fiber obtained in thisway, the segments being used as a measuring cell.

The invention claimed is:
 1. A particle detection apparatus comprising:a channel including an inlet and at least one channel wall, the inletpermitting light to be introduced into the channel, the or each channelwall enclosing a sub-wavelength bore that defines a channel path throughwhich light may propagate; a light source configured to introduce lightinto the channel via the inlet, the channel being shaped to guide thelight to propagate along the channel path for illuminating a particle ora plurality of particles located in the channel path; and a monitoringdevice configured to detect scattered light that is created by theillumination of the or each particle by the guided light and that leavesthe channel by passing through the or each channel wall.
 2. A particledetection apparatus according to claim 1 wherein the inlet is configuredas an open inlet.
 3. A particle detection apparatus according to claim 2wherein the open inlet is configured to permit the or each particle tobe introduced into the channel.
 4. A particle detection apparatusaccording to claim 1 wherein the inlet is configured as a lightincoupling inlet.
 5. A particle detection apparatus according to claim 1wherein the scattered light leaves the channel through the or eachchannel wall at a non-zero angle to a guided direction of the guidedlight.
 6. A particle detection apparatus according to claim 1 whereinthe monitoring device is configured to measure the coherent scatteringintensity of the scattered light, the incoherent scattering intensity ofthe scattered light, the spectrum of the scattered light, thedistribution of the scattered light over a plurality of directionsand/or the dynamic motion of the or each particle.
 7. A particledetection apparatus according to claim 1 wherein the monitoring deviceis configured to track the or each particle's motion through detectionof the scattered light.
 8. A particle detection apparatus according toclaim 1 wherein the monitoring device is configured to measure anemission spectrum of the or each particle.
 9. A particle detectionapparatus according to claim 1 wherein the monitoring device isconfigured to detect fluorescent light that is created by theillumination of the or each particle by the guided light and that leavesthe channel by passing through the or each channel wall.
 10. A particledetection apparatus according to claim 1 wherein the monitoring deviceis configured to detect coherently scattered light and/or incoherentlyscattered light, and optionally detect the spectrum of the coherentlyscattered light and/or incoherently scattered light, that is created bythe illumination of the or each particle by the guided light and thatleaves the channel by passing through the or each channel wall.
 11. Aparticle detection apparatus according to claim 10 wherein themonitoring device is configured to track the or each particle's motionthrough detection of the coherently scattered light and/or incoherentlyscattered light.
 12. A particle detection apparatus according to claim 1wherein the channel is arranged to convey at least one particle that issmaller than the wavelength of the guided light.
 13. A particledetection apparatus according to claim 1 wherein the channel is formedin or as: a waveguide; a chip-based platform, optionally alithographically formed chip-based platform; or a capillary.
 14. Aparticle detection apparatus according to claim 1 wherein the channel isformed in or as an optical fibre.
 15. A particle detection apparatusaccording to claim 14 wherein the optical fibre is a single-mode opticalfibre.
 16. A particle detection apparatus according to claim 14 whereinthe optical fibre is made from silica or polymer.
 17. A particledetection apparatus according to claim 1 wherein the particle detectionapparatus includes a driving mechanism for driving the or each particleto flow along the channel path.
 18. A particle detection apparatusaccording to claim 17 wherein the driving mechanism includes electrodes,which are incorporated into the channel and are configured to provide anelectrophoretic force to steer the or each particle along the channelpath.
 19. A particle detection apparatus according to claim 17 whereinthe driving mechanism is external to the channel and is configured togenerate an electric field so as to provide an electrophoretic force tosteer the or each particle along the channel path.
 20. A particledetection apparatus according to claim 17 wherein the driving mechanismis configured to drive the or each particle to overcome its or theirthermal Brownian motion when driving the or each particle to flow alongthe channel path.
 21. A particle detection apparatus according to claim1 wherein the monitoring device includes at least one light detectorand/or an imaging device configured to process the detected light so asto create an output signal corresponding to the or each particle.
 22. Aparticle detection apparatus according to claim 1 including a pluralityof channels, each channel including an inlet and at least one channelwall, each inlet permitting light to be introduced into the respectivechannel, the or each channel wall being arranged to define a respectivechannel path through which light may propagate.
 23. A particle detectionapparatus according to claim 22 wherein the monitoring device isconfigured to simultaneously detect scattered light that is created bythe illumination of the or each particle by the guided light and thatleaves each channel by passing through the or each corresponding channelwall.
 24. A particle detection apparatus according to claim 1 includinga plurality of light sources, the plurality of light sources beingconfigured to introduce light into the channel via the inlet.
 25. Aparticle detection apparatus according to claim 24 wherein the pluralityof light sources is configured to simultaneously introduce light intothe channel via the inlet.
 26. A particle detection apparatus accordingto claim 1 including a plurality of monitoring devices, the plurality ofmonitoring devices being configured to detect scattered light that iscreated by the illumination of the or each particle by the guided lightand that leaves the channel by passing through the or each channel wall.27. A particle detection apparatus according to claim 26 wherein theplurality of monitoring devices is configured to simultaneously detectscattered light that is created by the illumination of the or eachparticle by the guided light and that leaves the channel by passingthrough the or each channel wall.
 28. A particle detection apparatusaccording to claim 1 wherein the monitoring device is configured tomeasure the coherent scattering intensity of the scattered light, theincoherent scattering intensity of the scattered light, the spectrum ofthe scattered light, the distribution of the scattered light over aplurality of directions and/or the Einstein Stokes diffusion constant ofthe or each particle.
 29. A method of detecting at least one particle,the method comprising the steps of: providing a channel including aninlet and at least one channel wall, the inlet permitting light to beintroduced into the channel, the or each channel wall enclosing asub-wavelength bore that defines a channel path through which light maypropagate; providing a particle or a plurality of particles in thechannel path; introducing light into the channel via the inlet, thechannel being shaped to guide the light along the channel path andthereby illuminate the particle or the plurality of particles located inthe channel path; and detecting scattered light that is created by theillumination of the or each particle by the guided light and that leavesthe channel by passing through the or each channel wall.